Halogen containing lead activated calcium sulfide luminescent composition

ABSTRACT

An ultraviolet emitting, cathodoluminescent, halogen and lead activated calcium sulfide composition emits as a narrow band with a peak at about 365 nanometers, under cathode ray excitation, with an increased efficiency of cathodoluminescence over the presently used oxygen dominated phosphors which emit in this region. A preferred method of preparation is set forth in which the lead and halogen are added to calcium sulfide which is fired to produce the phosphor.

United States Patent Lehmann Dec. 9 1975 HALOGEN CONTAINING LEAD [56]References Cited ACTIVATED CALCIUM SULFIDE UNITED STATES PATENTSLUMINESCENT COMPOSITION 3,673,102 6/1972 Lehmann et al 252/301.4 s [75]Inventor: Willi Lehmann, Murrysville, Pa. FORElGN PATENTS ORAPPLICATIONS [73] Assignee: Westinghouse Electric Corporation, 650,4582/ 195! United Kingdom 252/3014 S Pmsburgh, OTHER PUBLICATIONS [22]Filed: Oct. [8, 1973 Lenard et al., Handbuch Der Experimental Physik,[21] APPL NO: 407,728 .1 Teil, 1928, pp. 342-345.

[44] Published under the Trial Voluntary Protest Primary ExaminerJackCooper Program on January 28, 1975 as document no. Attorney, Agent, orFirm-W. D. Palmer B 407,728.

Related US. Application Data [57} ABSTRACT [63] cgminuatiomimpan of SenN0 876 766 An ultraviolet emitting, cathodoluminescent, halogen H369abandoned and lead activated calcium sulfide composition emits as anarrow band with a peak at about 365 nanome- 30 Foreign A li fi prioritym ters, under cathode ray excitation, with an increased efficiency ofcathodoluminescence over the presently 1970 Japan 4593590 used oxygendominated phosphors which emit in this CL region. A preferred method ofpreparation 15 set forth Cl 2 in which the lead and halogen are added tocalcium 58 Field 5155a 252/3014 s sulfidfi which is fired ProducePhosphm' 4 Claims, 2 Drawing Figures US. Patent Dec. 9, 1975 WAVELENGTH,nm

FIG.I.

FIG.2.

,u. szconus HALOGEN CONTAINING LEAD ACTIVATED CALCIUM SULFIDELUMINESCENT COMPOSITION CROSS REFERENCE TO A RELATED APPLICATION Thisinvention is a continuation-in-part of copending application Ser. No.876,766, filed Nov. [4, 1969 now abandoned and entitled HalogenContaining Lead Activated Calcium Sulfide Luminescent Composition."

BACKGROUND OF THE INVENTION The cathodoluminescent composition of leadactivated calcium sulfide has been described in the literature, and isdescribed as emitting with an emission band in the green, blue, violetor ultraviolet. The use of sulfate, borate and halide fluxes has alsobeen described in the literature in preparing a variety of calciumsulfide luminescent compositions, although the use of halide with leadin calcium sulfide is not described. The most recent publicationdescribing lead activated calcium sulfide phosphors does not teachhalide in this phosphor (O. Sorge, Thesis presented at Technical Unionof Berling, 1959).

Fast decay cathodoluminescent phosphors with emission in thenear-ultraviolet are primarily used for flying spot scanners and similarcathode ray tube applications. The cerium-activated calcium-magnesiumsilicate phosphor, referred to hereinafter as P-l6 which is the RadioManufacturer Association designation, has remained the standard phosphorfor flying spot scanners for some years now. This phosphor exhibits abroad band emission peaking at about 380 nanometers, but extending fromabout 300-475 nanometers. The P-l6 phosphor is subject to permanentdamage under intense electron bombardment.

SUMMARY OF THE INVENTION It is an object of the invention to prepare acathodoluminescent phosphor which emits as a narrow band in the regionof 300 to 400 nanometers with a high efficiency.

It is a further object of the invention to provide an efficientcathodoluminescent phosphor which is resistant to electron bombardmentdamage.

It has been discovered that by incorporating chlorine, bromine or iodinein a lead activated calcium sulfide composition that this compositionexhibits a fast decay and is a very efficient cathodoluminescentphosphor emitter in a single narrow band in the near ultraviolet with apeak near 365 nanometers. It has also been discovered that, for maximumultraviolet emission that the phosphor should be substantially free fromalkali metal.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a comparison of the emissioncharacteristic of the cathodoluminescent phosphor of the presentinvention, curve A, as compared to the P-16 phosphor curve C, underequivalent cathode ray excitation, and also to the lead activatedcalcium sulfide phosphor which does not include the halide, curve B. Theemission intensity is here normalized so that peak heights are assigneda value of 100; the absolute peak intensities are of course not equal.

FIG. 2 is a plot of the decay of relative emission intensity againsttime for the phosphor of the present in- 2 vention after excitation byan electron beam pulse of 10 kilovolts and approximately a 10 microampper square centimeter current density.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The phosphor is preferablyprepared by adding to about 1 mol of calcium sulfide, 10" gram mol oflead sulfide, and 10 gram mol of calcium chloride. This raw mix afterbeing thoroughly mixed is fired in an inert or sulfurizing atmosphere atabout [200C for about 1 hour to render the compositioncathodoluminescent. The firing temperature and time are not critical butare preferably carried out at from I, l ,300C with the lower thetemperature the longer the firing time. A portion of the chlorine in theraw mix is incorporated into the final phosphor composition, in thepreferred embodiment about 10" gram-atom of chlorine per mol of calciumsulfide is incorporated.

A comparison of the emission spectra of cathodoluminescent phosphors isshown in FIG. 1 where all phosphors were excited by an electron beam of10 kilovolts and about 1 microamp per square centimeter current density.It can be seen that the phosphor of this invention composition emits asa single narrow emission band in the near ultraviolet with the peak atabout 365 nanometers and a half width of about 20 nanometers, incomparison to the prior art calcium sulfide lead activated compositionwhich does not contain halogen and whose emission characteristic underthe same cathodoluminescent excitation is designated curve B in thefigure. The emission of the standard P-l6 phosphor under the sameexcitation is plotted as curve C in the figure and is to be contrastedand compared with curve A representing the emission of the phosphor ofthe present invention. The emission of the lead and chlorine activatedcalcium sulfide phosphor of the present invention exceeds the emissionof the P-l6 phosphor by a factor of about 2.8 if the entire emissionspectrum is measured, and by a factor of about 3.5 is only that portionof the emission spectrum in the near ultraviolet which is effectivelyutilized is analyzed. A higher proportion of the P-l6 emission energyfalls in the region which is in effect unusable for the application towhich the phosphor is generally applied. Thus, it can be said that sincethe energy efficiency of the cathodoluminescent P-l6 phosphor is about 5percent, then the efiiciency of the phosphor preferred embodiment of theinvention is about 14 percent.

The amount of lead activator in the phosphor of the present inventioncan be readily varied to provide an atomic ratio of lead to sulfur offrom about 10 to 10 which thus provides an activating proportion of leadof from about 10 to 10 gram-atom of lead per mol of sulfide.

While in the preferred embodiment the lead was added to the compositionas lead sulfide other lead containing compounds can be utilized, such aslead acetate, lead nitrate.

It has been discovered by chemical analysis that the halogen isincorporated into the phosphor. For example, when about 10 gram mol ofcalcium chloride is added in a raw mix containing about 1 mol of calciumsulfide and 10 and gram-atom of lead are fired, only about 10- gram-atomof chlorine per mol of calcium sulfide is effectively incorporated intothe preferred composition, which means a lOzl atom ratio of halogen tolead. The remaining halogen is either volatilized away during the firingor is present as calcium halide which is readily dissolved by a waterwashing of the activated composition.

It has been discovered that only when the halogen is incorporated in agram-atom amount at least equal to and up to about ten times greaterthan the lead does the composition exhibit the intense, narrow bandultraviolet emission desired.

Each of the halogens, chlorine, bromine, and iodine can be utilized inthis composition, but chlorine yields the most efficient phosphor.

The halogen is preferably supplied in excess in the raw mix over theamount desired to be incorporated to facilitate the incorporation andalso promote crystal growth. In general it has been found preferable toprovide a halogen content in the raw mix of about 2 gram-atom pergram-ratio of sulfur in order to incorporate about l0 gram-ratio ofhalogen per mol of sulphide in the final composition.

While the halogen has been described as being added to the raw mix ascalcium halide, it may also be added as ammonium halide, or even byincluding a high halogen content in the firing atmosphere.

It has also been discovered that the ultraviolet emission is adverselyaffected by the inclusion of an alkali metal. When alkali metal isincluded in the raw mix in amounts which are of the same order ofmagnitude as the amount of halide, there is essentially no ultravioletemission. Thus it, for example, the raw mix has a molar ratio of alkalimetal to halide of 0.l oil or greater, there is no discernableultraviolet emission. For maximum 4 development of the ultravioletemission, the phosphor should be substantially free of alkali metal.

It can be readily seen from FIG. 2 that a typical emission decay for thephosphor of the present invention to a value of one-third, or about 37percent, of the initial intensity takes about 0.6 microsecond. Thisdecay time is not as fast as the decay time for the P-l6 phosphor but itcompares favorably and thus suggests the use of the phosphor of thepresent invention in flying spot scanners and other cathode-rayapplications. The phosphor of the present invention exhibits a greaterstability and burn resistance to intense electron beams than the Pl6phosphor.

I claim as my invention:

1. A luminescent composition which has an emission substantiallyconfined to the ultraviolet range of 300-400 NM which composition issubstantially free from alkali metal, said composition consistingessentially of CaSzPb, X, wherein Pb is present in an activatingproportion of from about 10 to 10 gram-atom per mol of sulfide, and X isat least one of the halogens, chlorine, bromine or iodine in a gram-atomamount which is at least equal to and up to about ten times greater thanthe Pb gram-atom content.

2. The composition specified in claim 1, wherein said halogen is presentin a gram-atom amount of about'ten times the Pb garm-atom gram-atom 3.The composition specified in claim 1, wherein said Pb is present in anamount of about 10" gram-atom per mol of sulfide.

4. The composition specified in claim 1, wherein said halogen ischlorine.

1. A LUMINSCENT COMPOSITION WHICH HAS AN EMISSION SUBSTANTIALLY CONFINEDTO THE ULTRAVIOLET RANGE OF 300-400 NM WHICH COMPOSTION IS SUBSTANTIALLYFREE FROM ALKALI METAL, SAID COMPOSITION CONSISTING ESSENTIALLY OFCAS:PB, X, WHEREIN PB IS PRESENT IN AN ACTIVATING PROPORRTION OF FROMABOUT 10**-5 TO 10**-3 GRAM-ATOM PER MOL OF SULFIDE, AND X IS AT LEASTONE OF THE HALOGENS, CHLORINE, BROMINE OR IODINE IN A GRAM-ATOM AMOUNTWHICH IS AT LEAST EQUAL TO AND UP TO ABOUT TEN TIMES GREATER THAN THE PHGRAM-ATOM CONTENT.
 2. The composition specified in claim 1, wherein saidhalogen is present in a gram-atom amount of about ten times the Pbgarm-atom gram-atom
 3. The composition specified in claim 1, whereinsaid Pb is present in an amount of about 10 4 gram-atom per mol ofsulfide.
 4. The composition specified in claim 1, wherein said halogenis chlorine.